The ultraviolet rays contained in sunlight have high energy and, hence, cleave carbon-carbon bonds of a polymer with the aid of oxygen or water, thereby deteriorating the polymer. Because of this, additives such as an ultraviolet absorber are incorporated into general polymers.
Ultraviolet absorbers include organic compounds and inorganic compounds. Examples of the organic ultraviolet absorbers include benzotriazole compounds and benzophenone compounds. Such organic compounds, however, are apt to be deteriorated by irradiation with ultraviolet rays and are hence deprived of the effect thereof with the elapse of time.
On the other hand, metal oxides such as zinc oxide and titanium oxide are known as inorganic ultraviolet absorbers. Although these metal oxides have the property of absorbing ultraviolet rays, the metal oxides become transparent within the visible light region as the particle size thereof decreases. Consequently, resin compositions which contain a metal oxide having a small particle size transmit visible light but block ultraviolet rays.
However, when the particle size of those metal oxides is reduced, the surface of the particles is activated and comes to have a photocatalytic activity. For example, such fine particles induce oxidation-reduction reactions. Of those metal oxides, titanium oxide has a high catalytic activity, and decomposes and deteriorates the matrix resin itself. In addition, metal oxide fine particles have a problem that it is difficult to disperse the particles in a matrix resin because the particles are fine particles and the poorly dispersed state impairs the transparency of the resin.
Techniques of treating the surface of metal oxide fine particles with a surface-treating agent, e.g., a silane coupling agent, are known. For example, in patent document 1, surface-treated zinc oxide fine particles which are highly transparent in the visible light region and exhibit high blocking properties in the ultraviolet region and which have no photocatalytic activity are produced by mixing a dispersion of zinc oxide fine particles in a solvent with a silane compound, subsequently heat-treating and drying the mixture, and disaggregating the resultant dry particles.
Also known is a method in which dispersion of fine particles is enhanced using a dispersant or using a bead mill or the like. For example, in patent document 2, a dispersant based on a polyether/phosphoric ester compound is used to enhance dispersion of fine particles. There also is a problem that when particles in a solid state are added to and dispersed in a solution, aggregates are apt to generate and the aggregation results in a decrease in transparency and an increase in haze. Patent document 3 discloses, as a measure for overcoming that problem, a technique in which a resin composition having excellent transparency is prepared by treating zinc oxide fine particles having a number-average particle diameter of 0.5 to 20 nm, with a specific silane compound to thereby prepare surface-modified zinc oxide fine particles which are in a liquid state and dispersing these fine particles directly in a resin.
Patent Document 1: JP-A-2009-120721
Patent Document 2: JP-A-2005-213482
Patent Document 3: JP-A-2008-297396